In the electrolysis of aqueous alkali metal halide solutions in electrolytic cells having a diaphragm or membrane separator, the applied voltage required is the total of the decomposition voltage of the compounds being electrolyzed, the voltage required to overcome the resistance of both the electrolyte and the electrical connectors of the cell, and the overpotential required to overcome the passage of current at the surface of the cathode and anode. Such overpotential is related to such factors as the nature of the ions being charged or discharged, the current density at the electrode surface, the base material from which the electrode is constructed, the surface formation of the electrode, i.e., whether the electrode is smooth or rough, the temperature of the electrolyte, and the presence of impurities in the electrolyte and the electrodes. At the present time, knowledge of the phenomenon of discharge overpotential is not fully understood. It has been observed that there is a characteristic overpotential for each particular combination of discharging ion, electrode, electrolyte, current density, etc.
Because of the large quantities of chlorine and caustic required by a modern society, millions of tons of these materials are produced, principally by electrolysis of aqueous solutions of sodium chloride, each year. A reduction of as little as 0.05 volts in the working voltage of a cell translates into a meaningful economic savings, especially in the light of today's increasing power costs and energy conservation measures. As a result, the electrochemical industry is constantly in search of means which will reduce the voltage requirements for such electrolytic processes.
The development of the dimensionally stable anode and coatings therefor have resulted in a reduction in the anode and cathode spacing within electrolysis cells, this advance resulting in a large reduction in the voltage since electrolyte resistance is reduced within the narrow space between the electrodes.
Cathodes for electrolysis are generally made of wire screening, perforated plate or steel mesh material because of the low cost of such material and its resistance to the caustic environment in the catholyte. Further, hydrogen embrittlement, a problem with valve metal substrates, is avoided.
Various coatings have been proposed for depositing on the cathode mesh which coating reduces the hydrogen discharge overpotential for the cathodic reaction.
Japanese patent application publication No. 6611, published Aug. 7, 1956, describes a coating for electrodes used in the electrolysis of water, which coating comprises an alloy mixture or nickel or a nickel compound and zinc, coating the surface of the electrodes. The zinc component of the alloy mixture is then leached from the coating to give a cracked and porous surface which is principally nickel, which coating results in a lowering of the hydrogen overpotential for the electrolysis of water.
Similarly, Hahndorff, U.S. Pat. No. 3,272,728, describes a method for producing activated electrodes for water electrolysis wherein a nickel-zinc alloy is electrodeposited on the electrode surface to a thickness of between 30 and 50 microns. The coating is then leached in a sodium hydroxide solution to remove the zinc component and leave a porous Raney nickel surface on the electrode. This porous surface results in a lowering of the total discharge overpotential for hydrogen and oxygen of approximately 0.2 to 0.3 volts.
Canadian Pat. No. 955,645, discloses a leached nickel-zinc electro-deposit on fuel cell anodes as the base for the chemical deposition of a noble metal catalyst.
Strasser, U.S. Pat. No. 3,941,675, describes a bipolar electrolytic cell having bipolar electrodes therein which are coated on their cathode side with a nickel-phosphorous coating, which coating acts to reduce the hydrogen overpotential at the cathode surface.
The difficulty with the above-disclosed cathode coatings is that they have a relatively limited life and, after a period of six months to two years, these coatings have deteriorated to a point where they no longer effect any reduction in the hydrogen overpotential. At that point, the electrolytic cells must be completely disassembled so that the cathodes may be removed and replaced with new, coated cathodes or so that the old, spent cathode coatings may be renewed. The economics of this procedure have precluded commercialization of these processes.